A "lookahead" dynamic range limiter to stop peaks above the set threshold value.

Unlike the Soft Clipping Limiter, Broadcast Limiter and LADSPA limiter, this limiter does not use wave shaping distortion to limit the peaks. It is a very fast compressor and is able to limit the maximum peaks without introducing high levels of harmonic distortion.

The limiter is very simple to use and requires little explanation, but for anyone that is unfamiliar with limiters, here's a few tips.

Limit to (dB): This control sets the maximum peak level. As peaks in the original audio approach this level the gain is reduced so as to prevent the peaks exceeding the set level.

Hold (ms): Holds the gain at the reduced level after a peak is detected so as to prevent the gain from "riding the waveform" which would cause harmonic distortion.Shorter Hold times allow the peaks to be tracked more accurately and the limiter will respond faster to the dynamics.If there are high levels of very low bass it will be necessary to increase the Hold time to avoid distortion. The default 10 ms hold time is sufficient for frequencies down to 100 Hz without distortion. To cleanly limit high amplitude, very low frequency bass (down to 50 Hz) the Hold should be increased to 20 ms. Setting the hold to 50 ms is sufficient right down to 20 Hz but the delay before the gain level "recovers" is likely to be too slow for most material.

Make-up Gain (0=No, 1=Yes): When enabled (default) the output is amplified by an amount equal to the "Limit" level. If the input audio has a peak level of 0 dB, the peak output level will also be 0 dB. When disabled the peaks are limited only.

Additional notes:

Stereo Tracks: As is normal for this type of effect, the left/right channels of a stereo track are processed independently.

Lookahead: This limiter looks ahead for peaks and will begin to change the gain just before the peak occurs. This ensures that all peaks, no matter how fast they occur, will be caught. The lookahead time is roughly a quarter of the hold time.

Knee: This limiter uses a relatively "soft knee" so as to avoid unnecessary distortion. That is, the amount of compression (compression ratio) progressively increases as the input gets louder. At the "Limit to" level the compression ratio is infinite (brick wall) which ensures that peaks will not exceed the limit.

Creative use: This limiter can be used on its own, or can be used to limit peaks after running a compressor that does not use lookahead (such as the SC4 LADSPA compressor). This can produce "crisper" compression than using a lookahead compressor such as the standard Audacity compressor or Chris's dynamics compressor.

Over 0 dB input: The input waveform should not exceed 0 dB. Over 0 dB input signals are illegal and will be hard clipped to 0 dB before processing with the limiter. If necessary, the Amplify or Normalize effects should be run before applying this limiter to ensure that the input does not exceed 0 dB.

OK, I've tried this with a few sources and it's simply AWESOME. It's great to maximize loudness of your tracks without the ugly distortion you get with digital clipping or hard limiting, and also without the extra harmonics that a soft-clipping limiter creates. It manages to avoid both because, just like Steve said, it works as an extremely fast compressor with lookahead, so while others would chop or 'shape' the wave below the threshold, it simply pushes the volume down without any perceivable pumping or other undesired side-effects. It is even good for 'brickwalling', if you first apply a conventional compressor to even the track's loudness. I can honestly say that I've found THE limiter that I was starving for. So thanks a lot Steve for your time and efforts! I salute you!

Although it is "soft" in the sense that it uses a soft knee (progressive compression ratio), it is "hard" in the sense that it provides infinite compression (brick wall) at the "Limit" level. In contrast the "Hard Limiter" produces hard clipping (a type of "wave shaping"), which can then be moderated by adding some of the original peaks back on (increasing the "Residual level" above zero) and / or mixing in part of the unprocessed audio (reducing the "Wet level"). Unfortunately, when softening the clipping with either of these methods, the output will probably exceed the "dB limit".

The thing that would be lost by replacing the Hard Limiter is the ability to "hard clip", though due to the distortion that hard clipping produces I'd say that this is not usually very useful. Where wave shaping is required, the sclimiter and Broadcast limiters will generally produce superior results than the Hard Limiter.

Where "conventional" limiting (fast compression) is required, this Limiter is the right tool and I'm not aware of any equivalent alternatives in Audacity.

Personally I don't think the names are really important as long as everything is properly documented. That's what's wrong, IMHO, with Audacity's "Leveler", a strange compressor that adds noticeable distortion without anyone knowing exactly what it does or how it works.

The thing that would be lost by replacing the Hard Limiter is the ability to "hard clip", though due to the distortion that hard clipping produces I'd say that this is not usually very useful. Where wave shaping is required, the sclimiter and Broadcast limiters will generally produce superior results than the Hard Limiter.

Agree. I don't think the Hard Limiter is much better than amplifying beyond the 0dB limit. The distortion is there anyway, so maybe it could be replaced by the sclimiter. You can get proper brickwalling with it without the ugly distortion of hard clipping. This Limiter should also be added, and I don't think including 2 different limiters is a problem, since the developers find it OK to bundle 2 compressors (I'm counting the Leveler here).

I just installed this to attempt to tame some loud applause in a relatively quiet acoustic recording, but either I'm doing something wrong or something isn't working right.

The level on the track is pretty low - music peaks around -17 dB and applause peaks around -11 dB. What I'd like to do is just barely touch the music peaks while bringing the applause down to that level, so I can then bring everything up without the applause causing clipping. However, stuff well below the peak is getting pushed *way* down. Ex:

I'm looking at 5 seconds of audio now, peak is -19.6 dB. In theory, running the limiter with "Limit to" set to -15 dB should cause the audio to barely be touched, if at all. However, if I run it (with Make-up Gain turned off), everything gets pushed down to -29.1 dB. Am I not using it as intended? "As peaks in the original audio approach this level" - how closely is "approach"?